Integrated circuits are typically manufactured by using a plurality of similar dies on a single large wafer which may be four to five inches in diameter. As many as several hundred dies may be fabricated on each wafer but due to imperfections in processing many of the dies produced on a wafer will be defective. A single die may have thousands of transistors and conducting lines and the failure of any one of these elements can cause the die to be defective. The percentage of good dies for a wafer is referred to as the yield. The cost per die is naturally lower when the yield is made higher.
Certain types of integrated circuits, such as semiconductor memories, have a plurality of similar circuits. It is possible to test a device of this type and determine exactly which one of the plurality of circuits is defective. The device can be manufactured to have a number of extra circuits without substantially increasing the size of the die. It is then possible to take a normally defective die and test the die to find the specific defective circuit. By performing operations upon the die the defective circuit can be disconnected and one of the extra circuits interconnected to operate in place of the defective circuit. This procedure makes it possible to "repair" many of the dies which would normally be discarded. If a significant number of the dies can thus be repaired the yield can be increased thereby decreasing the unit cost.
One procedure for removing a defective circuit and replacing it with an extra operable circuit is to burn-out circuit links of the die with the use of a laser beam. However, the laser beam must be precisely aigned and precisely focused in order to blow the desired link without harming nearby conductor lines and circuit elements. As the line dimensions for integrated circuits become smaller it becomes more difficult to properly focus the laser beam to blow the desired links. The procedure for focusing the laser beam must also be extremely fast in order to economically operate on the dies at a mass production rate.
Therefore, there exists a need for a method and apparatus for rapidly and accurately focusing a laser beam on the surface of an integrated circuit.